This invention relates to an electronic battery charger control circuit. More particularly, this invention relates to a control circuit for a battery charger of the type designed to recharge lead acid batteries such as used in supplying power to electric vehicles, and for terminating the charging operation whenever the batteries are fully charged.
There are two important reasons for not overcharging a battery. The first reason has to do with the cost of electricity. Obviously, if a battery charger is operated beyond the point where the battery is fully charged, it is wasting energy and money. On the other hand, it is important that the battery become fully charged, otherwise it will not be ready for a full day's operation.
The other reason for not overcharging the battery has to do with battery life. Once a battery has become fully charged, applying further current to the battery causes both hydrogen and oxygen to be released from the electrolyte through the electrolysis process. The oxygen combines with the lead in the positive plates causing them to become soft and tend to fall apart, thus shortening battery life.
One typical prior art battery charger control circuit is essentially a timer which applies charging current to the battery for a specified period of time. If it is assumed that a battery becomes 80% or 90% discharged during the course of a typical 8-hour shift, and if the battery and the battery charger capacity are properly selected, then an 8-hour charge will usually put the battery back into condition for the next shift. However, if the battery is only 20% discharged, then a shortened charging time would be indicated, and if it were 100% discharged, then a longer charging time would be required.
In another prior art device, a voltage relay is used to sense the battery voltage, and when it has reached a predetermined level, to terminate the charging cycle. If the battery is defective in some way, the battery voltage might never reach the predetermined value.
In still another type of battery charger, the rate of change of battery voltage is used as a measure of the state of charge. However, variations in charging voltage, or noise on the output of the battery charger would adversely affect this type of measurement device, thus either shortening or prolonging the charging cycle.